Speech compressor-expander with signal sample zero reset

ABSTRACT

In an improved speech compressor which provides frequency transformation by passing speech signals through an analog shift register and controlling the shift rate of the register with a varying period, the control of the sample period and output blanking is improved by automatically initiating reset as soon as a zero crossing of the signal is detected within the enable blank interval. Thus, discard of information in the line and refilling of the line with the next initial portion of the sample signal will occur as soon as the detected zero crossing within the enable period blanks the output and the effective blanked period will thereby be shortened to the extent that a zero crossing is detected prior to the regular termination point of the ramp controlled voltage.

United States Patent 1191 Eppler, Jr. I

SPEECH COMPRESSOR-EXPANDER WITH SIGNAL SAMPLE ZERO RESET Nov. 5, 1974 OTHER PUBLICATIONS Lee, Time Compression and Expansion of Speech by [75] Inventor: William G. Eppler, Jr., Westport, the sampling Method, Jaes 11/ 1972 PP- V Conn Primary Examiner-Alfred H. Eddleman [73] Assignee: Cambridge Research and Attorney, Agent, or FirmDike, Bronstein, Roberts,

Development Group, Westport, Cushmail & Pfllnd 57 ABSTRACT [22] Filed: Feb. 12, 1973 I 11 an improved speech compressor which provides [2]] Appl 331536 frequency transformation by passing speech signals through an analog shift register and controlling the [52] US. Cl. 360/26, 179/1 SA, 179/ 1 5.55 T, shift rate of the register with a varying period, the con- 360/7, 360/27 trol of the sample period and output blanking is im- [51] Int. Cl. Gllb 5/02, G1 1b 27/18 P ed y automatically initiating reset as soon as a [58] Field of Search..,, 179/1555 T, 1555 R, 1 SA, zero crossing of the signal is detected within the en- 179/100.2 T; 360/7, 8, 27, 25, 26 able blank interval. Thus, discard of information in the line and refilling of the line with the next initial por- [56] References Cited tion of the sample signal will occur as soon as the de- UNITED STATES PATENTS tected zero crossing within the enable period blanks 3 I04 284 9/1963 F h 179/15 55 T the output and the effective blanked period will 3480737 11/1969 22228 179mm 2 thereby be shortened to the extent that a zero crossing 3 62l l 11/1971 Pa asf 179/1555 T is detected Prior to the regular termination Point Of 3,632,877 1/1972 Gray 179/1 SA the p Controlled voltage- 1 ,68l,756 8/[972 Burkhard 179/1 SA 4 Claims 2 Drawmg Flgures AUDIO IN T T T f rT.1 I 1 i 46 casters 1 1 seats: 45 GENERATOR 65223011 DRIVERS fififilo n I l g (VCP) I (GAIN=|) 1 25 I es 1 I 52 1s 1 i 48 E 51 RESET I MOTOR f 50\ I ONE- LE. T

| JUUUL 1 that I Essen AND AUTO l i PULSES I RESET l F 7e| 12, J

SPEECH COMPRESSOR-EXPANDER WITH SIGNAL SAMPLE ZERO RESET BACKGROUND OF THE INVENTION This invention relates generally to sound or speech compressors in which a sound recording is reproduced at a speed different from the speed at which the sound was recorded and the frequency components of the recorded signal are restored to approximate those of the normal speech which was recorded thus permitting a recording to be played back with normal speech frequencies but at an elapsed time different from that in which the recording was made. If the playback trans port is run at a higher speed than the recording speed the time is compressed and the information is played back in less time than was taken to record it. If the transport is run at a slower speed than its recording speed the time is expanded and the information conveyed in a longer time than it took to record. In this application, compression will be used generically to mean both time compression and time expansion since the compression ratio C is greater than one for timecompression and a value between zero andless than one for time expansion.

CROSS-REFERENCE TO RELATED APPLICATION This application is related to the application of Murray M. Schiffman entitled IMPROVED SPEECH COMPRESSOR-EXPANDER filed with even date herewith the disclosure of which is hereby incorporated by reference.

SUMMARY OF THE INVENTION The present invention provides for improvement in the operation of a speech compressor expander of the class described in which a delay line is periodically con trolled by the delay thereof with the rate of change of delay being selected to provide the desired frequency transformation for the signal passing through the line. In such systems, the line must be periodically reset to an initial value and the portion of the signal stored in the line at this time discarded. Where the controlled delay line takes the form of an analog shift register having many stages through which the signal passes, it is necessary at the end of a sample period to refill the register by clocking in the speech input signal until the line is full so that at the beginning of the next sample processing period the processed speech signal will appear at the output of the line.

In adapting such systems for blanking the output at a detected zero crossing of the processed audio signal, it is necessary to provide an interval during which to look for the zero crossing. Thus initiation of blanking in response to the detection of a zero crossing may occur within such enabling interval at a random time with automatic reset of the sample period occurring at the end of some predetermined interval in the event that a zero crossing is not detected. The present invention provides for minimizing the sequential time intervals'involved in accomplishing the various functions by initiating reset of the" ramp control of the analog'shift register at the same time the output is blanked upon the detection of a zerocrossing thereby eliminating the additional delay in reset which would be involved if the 2. sample period were permitted to extend to the end point for" its periodic repetition. Witht'h'is arrangement, the gap between audible samplesis decreased on" a statistical basis and theover all audible effect is improved by reducing the interval required for resetting and reprocessing the system to the minimum possible requirement.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the signal processor and control circuit therefor in accordance with the invention.

FIGS. 2 (a)2(e) are waveform diagrams useful in explaining the operation of the system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, the general arrangement of the frequency processor and control circuit in a speech compression system of the type described in the hereinbefore referenced Schiffman application will be des'cribedi Manual control 17 selects motor transport speed at motor control circuit 44. The motor control circuit 44 provides an output which is applied to an f(c) circuit 45 which transforms the linear setting of C on control 17 to a voltage on line 41 which includes the function K c 1/0 l as the functional relation for establishing the ramp slope as a function of compression factor C. This input to ramp generator 42 produces on output line 46 a ramp voltage which has a predetermined repetition interval and slope determined by the input voltage on line 41. The ramp resets-from signal on line 53.

A blank enable circuit 43 determines whether the unit is in compression or expansion mode by sensing the voltage on line 47 relative to a fixed level such as 7.8 volts. When the voltage on line 47 is above 7.8 volts the unit is compressing speech for C greater than one and for this condition the blank enable circuit 43 will produce two outputs. By sensing an input on' line 48, derived from the output of ramp generator 42 on line 46, the blank enable circuitproduces on line 49 a blank enable signal when the input on line 48 crosses the three volt level and on line 51 produces areset signal when the signal on line 48 reaches the two volt level which is designated as the-end of the ramp period. If the end of the ramp period is reached by the ramp voltage reaching the 2 volt level, the reset pulse on line 51 will trigger a one shot 52 which is connected-on line 53 to reset the ramp generator 42 and at the end of the reset one shot pulse which resetsthe ramp generator 42 to an initial voltage (7.8 volts) the next ramp excursion begins. The length-of the reset one shot pulse from unit 52 is long enough to permit the ramp generator to rese the 7.8 volts as a starting point.

For time expansion, i.e., when C is less than one, the voltage signal on line 47 as applied to the blank enable unit 43 is compared with the E voltage on line 50, shown as 7.8 volts,-to generate a'ramp of opposite slop and substantially constant repetition interval in the ramp generator 42. Also a blank enable signal on line 49 as the ramp approaches its end value of 7.8 volts and the-end resetpulse on line 51 are produced.

The output V of ramp generator 42 is applied to a voltage controlled period generator 54 which generates on line 55 a square wave outputwhich has a pulse pe- 3 riod that varies linearly with the linear variation of the ramp voltage V The output on line 55 is applied to a phase splitting driver circuit 65 which produces on outputs 66 and 67 phase opposed square waves corresponding to thesquare wave input on line 55 which are the P P drives for shifting analog signals through a pair of analog shift registers 68 and 69.

The shift registers 68 and 69 are connected for processing disturbance cancellation introduced by the operation of the units at a varying shift pulse period. For this purpose the audio signal from the preamplifier appears at input terminal 71 of an amplifier 72 which has its output connected as an input to analog shift register 68 and also connected to an inverter 73. The output of the inverter 73 is applied as the input signal for analog shift regster 69. The outputs of the shift registers 68 and 69 are applied as the subtractive inputs to a differential unity gain amplifier at the input of an audio amplifier unit 74.

The just described circuit including the shift registers 68 and 69 and the amplifier 71 and inverter 72, together with the differential input amplifier 74 provides for frequency transformation of the input signal on line 71 and at the same time cancellation of the disturbances and distortion introduced by processing that signal through the shift registers 68 and 69.

The amplifier 74 further amplifies the difference signal obtained by comparing the two inputs from the shift registers 68, 69 and applies the amplified audio output signal on line 25. Also contained in the unit 74 is a zero crossing detector which provides on output line 75 a series of pulses representing the time of detection of zero crossings of the difference input signal.

A blank-unblank control circuit 76 receives the zero crossing pulses on line 75 and operates when enabled by the blank enable signal on line 49 to produce an output blanking signal on line 77 which blanks the output ofaudio amplifier 74. This arrangement blanks the output signal level on line to zero signal level during the reset period for the shift registers and ramp generator controlled portions of the circuit. The D-fiip-flop circuit 76 is responsive to a signal on line 78 applied to reset input to terminate the blanking output on line 77 at the next zero crossing pulse thereby permitting the signal on output line 25 to resume its signal level value.

The counter 81 produces an output on line 78 at the end of 256 counts corresponding to the length of the individual registers 68, 69 responsive to the pulses from line 55 applied on the counter input 82. When the counter 81 completes a count of 256 input pulses on line 82 it produces an output on line 78 that ends the blanking period pulse on line 77. A similar output to that on line 53 appearing on line 83 resets the counter 81. The interval timed by the counter 81 counting 256 counts of the pulse signal on line 55 provides blanking duration for the amplifier 74 sufficient to permit the pulses on line 55 after passing through drivers 65 to shift the registers 68, 69 and empty the contents thereof before unblanking occurs and audio output on line 25 is resumed. At the same time the shift pulse applied to the registers 68, 69 enters audio signal from terminal 71 through the amplifer and inverter 72, 73 so that the registers 68, 69 are full at the end of the blank ing interval and ready to supply audio samples to the input of amplifier 74 and resume the next signal period.

In the event that no zero crossing signal on line occurs curing the enable period established by the signal on line 49 prior to the occurrence of the reset signal on line 51 then the reset signal which is applied on line 83 to the counter 81 immediately causes a blank output signal on line 78 to the circuit 76 to blank the audio amplifier 74 by signal on line 77. Again, the action of the counter 81 removes the signal on line 78 toremove the blanking signal on line 77 at the end of the count of 256 pulses on input line 82. The audio output is thus blanked for the duration of the output pulse from reset one shot 52 and the period required to count 25 6 counts in counter 81 plus the length of time after the end of the 256 count for the detection of the next signal zero crossing.

The blank-unblank control logic 76 comprises a D- flip-flop. With the inputs indicated on lines 49, 75 and 78, the control of the output will be as follows. The enable input 49 permits blanking upon the occurence of a zero crossing within that interval and if it does not occur the reset flip-flop pulse on 83 forces the start of the blank interval which continues during the 256 counts in the counter 81. At the end of the 256 counts the signal on line 78 permits removal of the blanking signal which occurs at the next zero crossing detection signal on line 75 to switch to unblank signal on line 77.

For the purpose of resetting one shot 52 prior to the ramp end signal on line 51 and in synchronism with the detection of a zero crossing by blank control 76, the signal on line 77 is applied to pulse generator 101 to trigger reset one shot 52 upon the occurrence of a blank signal on line 77. The output of reset one shot 52 on line 53 thus resets the ramp generator 42 coincident with the detection of the first Zero crossing after blank enable and the appearance of such signal on line 77.

Referring now to FIG. 2, the operation will be described. In FIG. 2(a) a ramp signal at 46 falls to the 3- volt level established in blank enable circuit 43 to initiate the blank enable signal, FIG. 2(c), on line 49. At any time after the line 49 blank enable signal is initiated, a zero crossing of the audio signal, FIG. 2(b), will trigger the ramp reset, terminate blank enable and start the blank signal, FIG. 2(d). Thereafter, the timing provided by reset one shot 52 and the counting of 256 counts in couter 81 extends the blanking interval as previously described to initiate audio output upon the detection of a zero crossing immediately after the completion of the 256 count, FIG. 2(e). Without the triggering of the ramp generator provided by pulse generator 101 upon detection of the zero crossing and initiation of blanking, the gap involved in the timing of the reset and signal processing functions during this interval could be extended to thepoint 103 shown in FIG. 2(c) which is the normal termination of the blank enable pulse when the ramp 46 has fallen to the 2-volt level. Thus, the gap in the audio output signal will ordinarily be reduced by a significant amount in all cases except when no zero crossing is detected during the full duration of the blank enable signal.

Similar conditions occur for expansion where a blank enable signal occurs prior to the end of the ramp voltage period and a ramp end signal follows if no Zero crossing is detected.

While a specific embodiment for the purpose of the invention has been disclosed, it will be appreciated that other circuits to implement the function will be known to those skilled in the art and are to be understood as within the scope of the-invention as defined in the appended claims.

I claim:

1. In a speech compression-expansion system having variable delay line means repetitively varied to provide frequency transformation of signals passing through the line the combination comprising:

means for sensing zero crossing points on said signal at the output of said line;

enabling means for establishing an interval at the end of each repetitive variation;

blanking means responsive to detection of a zero crossing within said interval and if no such zero crossing is detected responsive to the end of said interval for blanking the output of said line; and

means responsive to the inititiation of said blanking for resetting said variation to start the next repetitive variation of said line.

2. Apparatus according to claim 1 in which said delay line means comprises an analog shift register with controlled shift period and said resetting includes initiation of shifting of signal samples into said register at the beginning of said blanking interval.

3. Apparatus according to claim 2 in which said blanking is maintained after the start of the next repetitive variation of said line at least until the shifting of signal samples into said register fills said register.

4. Apparatus according to claim 3 in which said blanking is terminated upon the detection of a zero crossing in said signal after said shifting fills said register. 

1. In a speech compression-expansion system having variable delay line means repetitively varied to provide frequency transformation of signals passing through the line the combination comprising: means for sensing zero crossing points on said signal at the output of said line; enabling means for establishing an interval at the end of each repetitive variation; blanking means responsive to detection of a zero crossing within said interval and if no such zero crossing is detected responsive to the end of said interval for blanking the output of said line; and means responsive to the inititiation of said blanking for resetting said variation to start the next repetitive Variation of said line.
 2. Apparatus according to claim 1 in which said delay line means comprises an analog shift register with controlled shift period and said resetting includes initiation of shifting of signal samples into said register at the beginning of said blanking interval.
 3. Apparatus according to claim 2 in which said blanking is maintained after the start of the next repetitive variation of said line at least until the shifting of signal samples into said register fills said register.
 4. Apparatus according to claim 3 in which said blanking is terminated upon the detection of a zero crossing in said signal after said shifting fills said register. 